Al insertion and additive effects on the thermoelectric properties of yttrium boride

The aluminoboride YxAlyB14 (x ∼ 0.57, 0.41 ≤ y ≤ 0.63) has been found to show striking p-n control of the thermoelectric properties through variations of the y occupancy of the Al site. The effect of Al was investigated in further extremes. Polycrystalline samples of Al-free YxB14(x ∼ 0.55; “YB25”) were successfully synthesized in sufficient amounts for bulk spark plasma sintering (SPS) samples and their thermoelectric properties were investigated. Y0.56Al0.57B14 was also prepared in comparison, and further Al was added to the samples through SPS treatment. We observed that Y0.55B14 exhibits large positive Seebeck coefficients, ∼1000 μV K−1, around room temperature and the absolute value of the Seebeck coefficient largely decreases with increase of temperature while that of Y0.56Al0.57B14 is proportional to T−1/2, indicating a strong effect of Al on the electronic structure around the Fermi level. Y0.55B14 was found to be strongly disordered with a relatively low thermal conductivity and short localizatio...